The invention relates to clipping voltage pulses produced by a frequency converter and particularly to clipping reflection pulses caused by the output voltage pulses of the frequency converter in connection with a motor.
In frequency converters provided with IGBT switches in particular the rates of change of output voltages are so great that the surge voltage fronts they cause, together with output filters restricting the rates of change, cause reflections that are due to the effect of cable and motor wave impedances of different magnitudes. At their worst, these reflections may appear at the motor terminals as voltage peaks of even almost double the magnitude of the intermediate circuit voltage of the frequency converter, and as the peaks proceed in the motor windings, they may cause partial discharges between the winding turns and, gradually, to dielectric breakdown of turn insulations and destruction of windings.
It is known in the prior art to cut some of the energy of such voltage peaks by means of a three-phase diode bridge, for example, for intermediate storage into a high-speed capacitor and to discharge it from there into the environment in the form of heat by means of a resistor. At high motor powers in particular the amount power to be discharged may rise to kilowatts and therefore specific power resistors are needed to perform the task, but the placement and cooling of the resistors may be problematic. Moreover, particularly in industrial motors that run almost uninterruptedly the clipping of peaks may consume considerable amounts of energy, especially since the clipping power is hardly at all dependent on the degree of loading of the motor.
WO 01/39347 discloses a solution in which the effects of excess voltages of a motor are reduced by means of a separate auxiliary wire to transfer energy away from the motor. This energy that is transferred away is compensated by means of a suitable passive connection or fed back in a rectified form to the intermediate circuit of the frequency converter. A problem with this solution arises from significant costs caused by the use of the separate auxiliary wire.
As is known from the prior art, motors fed with frequency converters require, almost without exception, one or more cooling fans, because a fan mounted to the motor shaft is not sufficient to cool the motor at low revolutions. These cooling fans, which are typically provided with an induction motor, are usually fed from the same AC network as the frequency converter. However, if the supply voltage is high, for example 690 volts, relatively low power (≦2.2 kW) fan motors, have often been fed with a lower voltage, for example 400 volts, for reasons of winding durability. This in turn requires the use of heavy, bulky and expensive auxiliary voltage transformers